The fabrication process of an electronic device often includes bonding a die onto a carrier substrate or a circuit board. In some cases, the orientation of the die with respect to the other components on the carrier substrate or the circuit board is important as the electronic device may not work if the orientation of the die is incorrect. Therefore, to ensure that the die is bonded in a correct manner, the orientation of the die is usually determined prior to the bonding process.
To facilitate the determination of the orientation of the die, certain types of dies, such as an RGB LED die, may include orientation features (or polarity mark features) indicative of the orientation of the die. Several prior art methods have been developed to determine the orientation of a die by inspecting the orientation features included in the die. These usually involve capturing an image of the die using a camera and inspecting the orientation features shown in the image. For certain types of dies, the orientation features are located on a top surface of the die and an image captured by a down-look camera above the die can show the orientation features quite clearly. The orientation of such a die can thus be determined in a relatively straightforward manner. However, for some other types of dies (for example, a flip chip LED die having a size ranging from about 3×5 mil to about 5×9 mil), the orientation features may be located on a bottom surface of the die or within the die. In these cases, it may be more difficult to capture a clear image of the orientation features as the line of vision between the camera and the die may be blocked by the other parts of the die and/or by the tape on which the die is mounted.
FIG. 1A shows examples of respective LED dies 100 having first, second and third orientation features 102b, 104b, 106b within the dies 100. In particular, FIG. 1A shows a top view of the LED dies 100 as captured by an optical microscope and FIG. 1B shows a cross-sectional view of one LED die 100. As shown in FIG. 1A, the LED dies 100 include a red LED 102, a green LED 104 and a blue LED 106. The red LED 102 includes a pair of first electrodes 102a and the first orientation feature 102b, the green LED 104 includes a pair of second electrodes 104a and the second orientation feature 104b, and the blue LED 106 includes a pair of third electrodes 106a and the third orientation feature 106b. The first, second and third orientation features 102b, 104b and 106b are indicative of the orientations of the red LED 102, the green LED 104 and the blue LED 106 respectively. As more clearly shown in FIG. 1B, the LED die 100 includes an epitaxy layer 108 arranged over the electrodes 102a, 104a, 106a. In the LED dies 100 as shown in FIG. 1A, the orientation features 102b, 104b, 106b are located within the die 100, in particular, within the epitaxy layer 108 (although, in other types of LED dies, the orientation features 102b, 104b, 106b may be located within the electrodes 102a, 104a, 106a respectively). The LED die 100 further includes a sapphire layer 110 on which the epitaxy layer 108 is arranged. The electrodes 102a, 104a, 106a are in the form of metal layers which are opaque. The orientation features 102b, 104b, 106b are also substantially opaque. On the other hand, the epitaxy layer 108 and the sapphire layer 110 are translucent.
FIG. 2A shows a cross-sectional view of a prior art apparatus 200 configured to determine an orientation of the LED die 100. Referring to FIG. 2A, the prior art apparatus 200 includes a down-look camera 202 and a top-down lighting module 204 attached to a side of the down-look camera 202. In use, the LED die 100 is mounted on a mylar tape 206 with the electrodes 102a, 104a, 106a in contact with the mylar tape 206. The mylar tape 206 is conveyed to move the LED die 100 to a position below the down-look camera 202 as shown in FIG. 2A. While the LED die 100 is at this position, the lighting module 204 projects coaxial light rays 208 (e.g. white light rays) downwards onto the LED die 100. This serves to provide general lighting so that the top surface of the mylar tape 206 and the general outlines of the LED die 100 are visible to the down-look camera 202. Light rays from the lighting module 204 are reflected by the orientation features 102b, 104b, 106b to reach the down-look camera 202 and an image of the LED die 100 is captured by the down-look camera 202. FIG. 2B is an example of images 220 of the LED die 100 captured by the down-look camera 202 of the apparatus 200. As shown in FIG. 2B, the images 220 contain a high amount of noise and the orientation features 102b, 104b, 106b are hardly visible. This is because a large part of the light rays reflected by the orientation features 102b, 104b, 106b are refracted by the sapphire layer 110 of the die 100 and/or scattered at the interface between the sapphire layer 110 and the epitaxial layer 108 due to geometric effects (see for example reflected light ray 210). Therefore, these light rays do not reach the down-look camera 202. The intensity of light reaching the down-look camera 202 is hence much lower as compared to the intensity of light originating from the lighting module 204. Accordingly, the captured image 220 is of a poor quality. Thus, it can be difficult to accurately determine the orientation of the die 100 using the prior art apparatus 200. Further, the orientation features 102b, 104b, 106b are usually small as compared to the size of the respective LEDs 102, 104, 106. For example, each LED 102, 104, 106 may have a cross-sectional area of about 220 μm×130 μm, whereas each orientation feature 102b, 104b, 106b may have a diameter ranging from about 40 μm to about 60 μm. This increases the difficulty in inspecting the orientation features 102b, 104b, 106b in noisy images.